A laying vessel normally comprises an assembly line for joining pipes into an underwater pipeline; and a laying ramp for laying the pipeline as the pipes are joined. Underwater pipelines are normally laid on the bed of a body of water, are used for conducting hydrocarbons, and extend for hundreds of kilometers. The pipes the pipeline is made from are normally of standard 12-meter lengths, which means the laying vessel can only carry a small percentage of the pipes needed to construct the pipeline, and must be supplied periodically with pipes from a carrier vessel. The hold of the carrier vessel is open at the top and designed to house the pipes, which are transferred from the carrier vessel to the laying vessel by positioning the carrier vessel alongside the laying vessel, and lifting and transferring the pipes by means of a crane installed on the laying vessel and equipped with a pipe gripping device. The crane normally has a reach covering the whole of the carrier vessel hold. And, because the crane employs hoisting cables and the swinging movement of the gripping device makes it difficult to position accurately, the gripping device must be positioned and attached to the pipes by workers inside the hold.
The above transfer method has various drawbacks, particularly when working in rough water. Firstly, the rougher the water, the more hazardous the working conditions inside the hold; so much so that it is standard practice to suspend transfer operations over and above a given wave height.
Secondly, transfer is relatively slow and subject to stoppages in rough water conditions.